As known in the art, structures may be bonded together by means of linear friction welding. In such a process, a surface on one of the structures is contacted (interfaced) to a surface on the other structure. The interfacing surfaces typically have complimentary features, i.e. similar lengths and similar widths. The two parts are rubbed together, in a back and forth, somewhat linear type oscillatory manner. The axis of the oscillation is typically roughly aligned with the longitudinal (lengthwise) axis of the interface, i.e. end to end. As the parts are rubbed, compressive force is applied to place the interface under high pressure. At the interface, frictional heat is generated and material from each part changes to a molten or preferably to a plastic state. Some of this material flows out from between the parts (flash flow), resulting in gradual decrease in the thickness, i.e. the dimension in the direction in which pressure is applied (the dimension perpendicular to the interface) of the parts. When the process is terminated, flash flow ceases, and at the interface, the remaining plastic state material of each part cools and changes back to solid state, forming bonds therein and bonding the two parts together.
However, a problem exists with this process in that the bond is usually incomplete, i.e. defective, at the ends of the interface. The nature of the defect is that of a void, or notch. It occurs, in part, because the ends of the interface, roughly on the axis of oscillation, are alternatingly exposed to ambient during each oscillation cycle. While exposed, the end is not rubbed and therefore not frictionally heated. Thus, as a result of the alternating exposure, the ends are only alternatingly heated and the temperature of the ends does not get high enough to produce complete bonding.
Efforts have focused on developing processes which insure that the defect does not form within the outline of the final shape of the product. In the fabrication of original equipment, part geometries can be oversized so that the defects which form are located outside the outline of the final product. The defects are then removed as the product is machined down to its final shape. However, in repair situations, a damaged portion is removed, but the remaining portion is already at its final shape and dimension, and therefore, an oversized geometry is not a viable alternative.
One of the numerous applications for linear friction welding is that of attaching blades (airfoils) to a rotor and thereby forming an integrally bladed rotor (IBR). In such an application, a base surface on the blade is interfaced to a slightly elevated surface on the rotor. However, without preventative measures, the bond risks being defective at the blade edges, because the blade edges are situated at the ends of the interface, roughly on the oscillation axis, and therefore, the blade edges are alternatingly exposed to ambient and only alternatingly heated during oscillation. As a result, the edge temperature does not get high enough to produce complete, adequate bonding. Although the defect may not constitute a crack per se, it could mature into such during engine operation, and thus, its presence in an IBR is unacceptable.
In the prior art approach for preventing defects at the edges for IBR repairs, the damaged portion of a blade under repair is removed, e.g., by removing a longitudinal section, and flanges, or collars, are provided around the edges of the remaining portion. A pair of jaws grip the remaining (undamaged) portion, (having, as its final shape, the final shape of the repaired part), and its associated flange to secure each in place for linear friction welding. Although the undamaged portion is already joined to the IBR (as by linear friction welding), the undamaged portion is not rigid enough, side to side, and too highly cantilevered to undergo linear friction welding without the support of the jaws or similar tooling. Similar flanges and jaws are also provided to grip a replacement portion, also having as its final shape, the final shape of the repaired part, to linear friction weld it to the undamaged portion. The flanges around each portion prevent the blade edges of the other portion from being alternating exposed to ambient; thus sufficient heat is generated to achieve effective bonding. Defects may be formed in the flange region, because the flanges may be alternatingly exposed to ambient, but the flanges are subsequently machined away, along with such defect.
However, with this prior art approach, it is difficult for the jaws to securely hold the blades without causing damage to the finished shapes. First, the jaws generally cannot conform exactly to the shape of an individual blade, because the shapes of the blades vary somewhat from one another, due to normal manufacturing inaccuracies, and therefore, the jaws may cause physical damage to any particular blade when they grip it tightly. Further, high performance blades often comprise titanium, a relatively soft metal which is relatively easily marred on its surface. Such marring is highly undesirable because it adversely affects the aerodynamic performance of the blades. Still further, the jaws typically do not have the same material composition as the blades, and consequently, they can leave residual chemical deposits which thereby contaminate the surface finish. The surface finish is so critical in some applications that during fabrication, often times gloves are worn when handling the blades in order to prevent contamination of the blade surface finish.
In addition, the replacement portion is almost certain to be significantly damaged because it is solely supported by jaws gripping its finished airfoil. Unlike the remaining portion, the replacement portion is not integral to a larger structure, such as the blade disk, which provides support. Thus, the jaws grip the finished airfoil securely enough to withstand tens of thousands of pounds of pressure, and therefore, almost certainly inflicts deep imprints in the airfoil.
Another problem with prior art approaches is that the flanges tend to be large, contributing as much, or more, surface area to the interface, as that of the blade. While this may help prevent defects from forming in the blade edges, it requires excessive process input energy to overcome the friction contributed by the flanges alone, and subjects the flanges to tremendous loads during linear friction welding, making them extremely difficult to hold securely, especially in view of the fact that the same pair of jaws has to grip the both the flanges and the blade. Prior art flanges also have sharp, orthogonal edges, making the flanges more susceptible to stress and cracking during linear friction welding.